New Printing Technology Uses Sound Waves To Control Liquid Droplets

New acoustophoretic printing uses sound waves to precisely create droplets from a variety of liquids.

The method independent of viscosity and composition of liquid.

The lower the amplitude of the sound wave, the bigger the droplet size.

It can be used to manufacture several new biopharmaceuticals and cosmetics.

The origin of home printing technology dates back to the 1930s. Today, we’ve liquid inkjet and laser printer technology that’s fast, manageable and energy efficient. At low-cost, they can provide high-quality output in vivid color.

To date, inkjet printing is the most common method used to precisely produce and pattern droplets. However, it only works with liquids that have at least 10 times greater viscosity than water.

Laser-induced forward transfer or valve-based printing is also an option but in these techniques, the volumetric transfer rate relies on nozzle size, source-substrate distance and material viscosity. One needs to adjust these parameters for each ink composition – an extremely difficult task for material whose properties change with temperature and time.

Many fluids that are crucial for bioprinting and biopharmaceuticals have at least 100 times greater viscosity than water. In fact, some sugar-associated biopolymers are as viscous as honey, which has nearly 25,000 times more viscosity than water.

To overcome these limitations, researchers at Harvard University have built a new technique that utilizes sound waves to precisely create droplets from liquids. It’s independent of viscosity and composition of the liquid. Basically, they’ve harnessed acoustic forces to print a variety of materials in a drop-on-demand manner.

How Does It Work?

Although any liquid can drip under gravitational force, gravity can’t alone control the droplet size and drop rate. For instance, water drips from a faucet in seconds, whereas Pitch that’s 200 billion times more viscous than water, creates a single drop in 10 years.

The team used sound waves to control drop formation. Such pressure waves have been tested in many experiments, including acoustic levitation, to defy gravity. Now it’s being used to assist gravity. This new technology is dubbed as acoustophoretic printing.

Credit: Daniele Foresti/Jennifer A. Lewis/Harvard University

They have designed a subwavelength acoustic resonator, which is capable of producing an extremely confined acoustic field that results in 100 times more pulling force than gravity (1G) at the nozzle tip. To put this into perspective, this is 4 times greater than the Sun’s gravitational force.

When the droplet reaches a particular size, the acoustic field pulls it off from the nozzle and ejects it towards the target position. The amplitude of the sound wave decides the size of droplets, irrespective of fluid’s viscosity: the lower the amplitude, the bigger the droplet size.

Testing and Applications

Using acoustophoretic printing with liquid metal to pile drops on top of another | Credit: Daniele Foresti/Jennifer A. Lewis/Harvard University

The team tested their technique on several materials, including biopolymers, honey, optical resins, stem-cell inks and liquid metals. Since sound waves can’t pass through droplets, the technique is safe enough to be used with sensitive biomolecules like proteins and living cells.